Low temperature synthesis of garnet solid state electrolytes: Implications on aluminium incorporation in Li7La3Zr2O12

Li ion conducting garnet electrolytes are attracting considerable interest for potential use in all solid state batteries. Nevertheless, their synthesis can be challenging due to the high temperatures required leading to significant Li loss, and consequently the need to add excess Li to counteract this. In this work, we report a low temperature biopolymer sol-gel route to synthesise these garnet materials using Agar (to ensure homogeneous mixing and nucleation through this biotemplating matrix), with the formation of the garnet phase starting at temperatures as low as 600 degrees C, with single phase samples of tetragonal Li7La3Zr2O12, and cubic Li6.4Al0.2La3Zr2O12 prepared at 700 degrees C (similar to 400 degrees C lower than the conventional solid state routes). Significantly, this route also allowed the synthesis of these garnets without the need for Li excess for the first time, due to the low temperature limiting Li loss. Moreover, if Li excess was used in the synthesis of cubic Li6.4Al0.2La3Zr2O12, Al incorporation was not observed at this temperature, and rather tetragonal Li7La3Zr2O12 was obtained. Contrary to previous assumptions, this indicates that the Li is more stable in the structure than Al at low temperature. Thus, Al incorporation only occurs if there is a deficiency of Li in the starting material, or if the sample is heated to elevated temperatures to induce Li volatility, so as to drive the incorporation of Al to charge balance the resultant Li loss.